606 



COLLEGE ZOOLOGY 



egg of the acorn worm (hemichordate) 

 develops into a small larva called a tornaria 

 (Fig. 429), which floats in the sea, is trans- 

 parent, has a bilateral symmetry, and is pro- 

 vided with bands of cilia for locomotion. In 



habitat and structure, this larva corresponds 

 almost exactly to a larval stage of the star- 

 fish and other echinoderms. This similarity 

 leads to the possibility that a form resem- 

 bling these larvae was the very remote an- 



Glia 



Mouth 



Anus 



Adult 0>» 



Bipinnaria of starfish 



Tornaria of acorn worm 



Figure 429. Comparison of the starfish (echinoderm) and acorn worm (hemichordate) larvae. 

 Note the striking structural similarities of the two. 



cestor of both the echinoderms and the 

 chordates, and that the lineal descendants 

 of this hypothetical ancestor chose two 

 paths, one of which, according to Wilder, 

 leads to the echinoderms and the other to 

 the acorn worms, tunicates, amphioxus, and 

 eventually the vertebrates. 



The question of the ancestry of the chor- 

 dates is not solved by accepting their rela- 

 tionship to the hemichordates, since this 

 latter group holds an uncommonly isolated 

 position; in fact, as previously mentioned, 

 some zoologists prefer these in an independ- 

 ent invertebrate phylum. Only from the 

 structure of the acorn worm larva can there 

 be concluded a distant connection with the 

 echinoderms. We must resign ourselves to 

 the thought that at the present time we are 

 not in a position to assert from what an- 

 cestral form the chordates, and with them 

 the hemichordates, were derived. The origin 



of the vertebrates is lost in the obscurity of 

 forms unknown to us. 



RELATIONS BETWEEN 

 CLASSES OF VERTEBRATES 



Anatomic and paleontologic investigations 

 are continually changing our ideas regard- 

 ing the interrelations of the vertebrates, and 

 we can indicate only provisionally the possi- 

 ble line of descent of the vertebrates and 

 the relations of one group to another. 



Figure 430. Facing page, a simplified family tree 

 of the principal animal phyla. The lines indicate 

 possible relationships. Evidence suggests an ancestral 

 two-layered type as the basic stock above the proto- 

 zoan stock. Relationships among other invertebrates 

 are uncertain. There are no satisfactory clues as to 

 the actual relationship between invertebrate and 

 vertebrate animals. Like all such diagrams, this one 

 reads from the bottom upwards. 



